function y = apply_operator(v)
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n = length(v)-1;
h = 1/n; hsq=h*h;

y = zeros(n+1,1);
y(2:n) = (-v(1:n-1)+2*v(2:n)-v(3:n+1))/(h^2);
y(1) = 0; y(n+1) = 0;